Sub-assemblies and processes for the production of electro-optic displays

ABSTRACT

An electrophoretic medium consists of a plurality of discrete droplets ( 104 ) in a polymeric binder ( 106 ′): each droplet ( 104 ) itself has a plurality of electrophoretic particles dispersed in a suspending fluid. An encapsulated electrophoretic display is produced by laminating the electrophoretic medium to a backplane ( 110 ) having at least one electrode at a temperature such that the polymeric binder ( 106 ′) flows and secures the electrophoretic medium to the backplane ( 110 ).

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of copending application Ser. No.10/904,063 (Publication No. 2005/0105159), which itself is acontinuation-in-part of copending application Ser. No. 10/249,957, filedMay 22, 2003 (Publication No. 2004/0027327, now U.S. Pat. No.6,982,178), which claims benefit of (a) Application Ser. No. 60/319,300,filed Jun. 10, 2002; and (b) Application Ser. No. 60/320,186, filed May12, 2002.

The aforementioned application Ser. No. 10/904,063 is also acontinuation-in-part of copending application Ser. No. 10/605,024, filedSep. 2, 2003 (Publication No. 2004/0155857), which claims benefit of (c)Application Ser. No. 60/319,516, filed Sep. 3, 2002.

The aforementioned application Ser. No. 10/904,063 also claims benefitof (d) copending Application Ser. No. 60/481,553, filed Oct. 24, 2003;(e) copending Application Ser. No. 60/481,554, filed Oct. 24, 2003; (f)copending Application Ser. No. 60/481,557, filed Oct. 24, 2003; (g)copending Application Ser. No. 60/481,564, filed Oct. 27, 2003; and (h)copending Application Ser. No. 60/520,226, filed Nov. 14, 2003.

This application is related to copending application Ser. No.10/064,389, filed Jul. 2, 2002 (Publication No. 2003/0025855), andclaiming priority of Provisional Application Ser. No. 60/304,117, filedJul. 9, 2001.

This application is related to (a) copending application Ser. No.10/145,861, filed May 13, 2002 (Publication No. 2002/0180688, now U.S.Pat. No. 6,864,875), which is a continuation of application Ser. No.09/436,303, filed Nov. 8, 1999 (and now abandoned), which in turn is adivisional of application Ser. No. 09/289,507, filed Apr. 9, 1999, whichclaims benefit of Application Ser. No. 60/081,362, filed Apr. 10, 1998;(b) copending application Ser. No. 10/687,166, filed Oct. 16, 2003(Publication No. 2004/0136048), which claims benefit of Application Ser.No. 60/419,019, filed Oct. 16, 2002, and which a continuation-in-part ofcopending application Ser. No. 08/983,404, filed Mar. 26, 1999, which isthe United States National Phase of International Application No.PCT/US96/12000, filed Jul. 19, 1996, which is itself acontinuation-in-part of application Ser. No. 08/504,896, filed Jul. 20,1995 (now U.S. Pat. No. 6,124,851); (c) copending application Ser. No.10/065,617, filed Nov. 4, 2002 (now U.S. Pat. No. 6,721,083), which is acontinuation-in-part of application Ser. No. 10/054,721, filed Nov. 12,2001 (published under No. 2002/0145792, now U.S. Pat. No. 6,538,801),which itself is a continuation-in-part of application Ser. No.09/565,417, filed May 5, 2000 (now U.S. Pat. No. 6,323,989), whichitself is a continuation-in-part of application Ser. No. 09/471,604,filed Dec. 23, 1999 (now U.S. Pat. No. 6,422,687), which is a divisionalof application Ser. No. 08/935,800 filed Sep. 23, 1997 (now U.S. Pat.No. 6,120,588). Application Ser. No. 08/935,800 claims priority fromProvisional Application Ser. No. 60/035,622, filed Sep. 24, 1996, and isalso a continuation-in-part of International Application No.PCT/US96/13469, filed Aug. 20, 1996, which itself claims priority fromProvisional Application Ser. No. 60/022,222, filed Jul. 19, 1996.Application Ser. No. 09/565,417 also claims priority from ProvisionalApplication Ser. Nos. 60/132,644 and 60/132,643, both filed May 5, 1999,and Provisional Application Ser. No. 60/134,245, filed May 12, 1999.Application Ser. No. 10/054,721 also claims priority from ApplicationSer. No. 60/254,342, filed Dec. 8, 2000. Finally, application Ser. No.10/065,617 claims priority from Application Ser. No. 60/350,735, filedNov. 12, 2001.

The entire contents of the aforementioned applications are hereinincorporated by reference. The entire contents of all United Statespatents and published and copending Applications mentioned below arealso herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to electro-optic displays and to processesand components for the production of such displays. As will be apparentfrom the description below, some aspects of the present invention arerestricted to electrophoretic displays, while other aspects can make useof other types of electro-optic displays. More specifically, thisinvention relates to (a) electro-optic media and displays with a binderwhich can also serve as a lamination adhesive; (b) processes for formingflexible displays; (c) color electro-optic displays; (d) processes andcomponents for forming electro-optic displays; and (e) processes formanufacturing a hybrid display formed from materials having differingcoefficients of thermal expansion.

In the displays of the present invention, the electro-optic medium (whena non-electrophoretic electro-optic medium) will typically be a solid(such displays may hereinafter for convenience be referred to as “solidelectro-optic displays”), in the sense that the electro-optic medium hassolid external surfaces, although the medium may, and often does, haveinternal liquid- or gas-filled spaces, and to methods for assemblingdisplays using such an electro-optic medium Thus, the term “solidelectro-optic displays” includes encapsulated electrophoretic displays,encapsulated liquid crystal displays, and other types of displaysdiscussed below.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin published U.S. Patent Application No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071; 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International Application Publication No. WO01/27690, and in U.S. Patent Application 2003/0214695. This type ofmedium is also typically bistable.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,727,881; 6,738,050; 6,750,473; and6,753,999; and U.S. Patent Applications Publication Nos. 2002/0019081;2002/0021270; 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770;2002/0130832; 2002/0131147; 2002/0171910; 2002/0180687; 2002/0180688;2002/0185378; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0038755;2003/0053189; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717;2003/0151702; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398;2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422;2004/0105036; 2004/0112750; and 2004/0119681; and InternationalApplications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO01/08241; WO 03/107,315; WO 2004/023195; and WO 2004/049045.

Known electrophoretic media, both encapsulated and unencapsulated, canbe divided into two main types, referred to hereinafter for convenienceas “single particle” and “dual particle” respectively. A single particlemedium has only a single type of electrophoretic particle suspended in asuspending medium, at least one optical characteristic of which differsfrom that of the particles. (In referring to a single type of particle,we do not imply that all particles of the type are absolutely identical.For example, provided that all particles of the type possesssubstantially the same optical characteristic and a charge of the samepolarity, considerable variation in parameters such as particle size andelectrophoretic mobility can be tolerated without affecting the utilityof the medium.) When such a medium is placed between a pair ofelectrodes, at least one of which is transparent, depending upon therelative potentials of the two electrodes, the medium can display theoptical characteristic of the particles (when the particles are adjacentthe electrode closer to the observer, hereinafter called the “front”electrode) or the optical characteristic of the suspending medium (whenthe particles are adjacent the electrode remote from the observer,hereinafter called the “rear” electrode (so that the particles arehidden by the suspending medium).

A dual particle medium has two different types of particles differing inat least one optical characteristic and a suspending fluid which may beuncolored or colored, but which is typically uncolored. The two types ofparticles differ in electrophoretic mobility; this difference inmobility may be in polarity (this type may hereinafter be referred to asan “opposite charge dual particle” medium) and/or magnitude. When such adual particle medium is placed between the aforementioned pair ofelectrodes, depending upon the relative potentials of the twoelectrodes, the medium can display the optical characteristic of eitherset of particles, although the exact manner in which this is achieveddiffers depending upon whether the difference in mobility is in polarityor only in magnitude. For ease of illustration, consider anelectrophoretic medium in which one type of particles is black and theother type white. If the two types of particles differ in polarity (if,for example, the black particles are positively charged and the whiteparticles negatively charged), the particles will be attracted to thetwo different electrodes, so that if, for example, the front electrodeis negative relative to the rear electrode, the black particles will beattracted to the front electrode and the white particles to the rearelectrode, so that the medium will appear black to the observer.Conversely, if the front electrode is positive relative to the rearelectrode, the white particles will be attracted to the front electrodeand the black particles to the rear electrode, so that the medium willappear white to the observer.

If the two types of particles have charges of the same polarity, butdiffer in electrophoretic mobility (this type of medium may hereinafterbe referred to as a “same polarity dual particle” medium), both types ofparticles will be attracted to the same electrode, but one type willreach the electrode before the other, so that the type facing theobserver differs depending upon the electrode to which the particles areattracted. For example suppose the previous illustration is modified sothat both the black and white particles are positively charged, but theblack particles have the higher electrophoretic mobility. If now thefront electrode is negative relative to the rear electrode, both theblack and white particles will be attracted to the front electrode, butthe black particles, because of their higher mobility, will reach itfirst, so that a layer of black particles will coat the front electrodeand the medium will appear black to the observer. Conversely, if thefront electrode is positive relative to the rear electrode, both theblack and white particles will be attracted to the rear electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the rear electrode,leaving a layer of white particles remote from the rear electrode andfacing the observer, so that the medium will appear white to theobserver: note that this type of dual particle medium requires that thesuspending fluid be sufficiently transparent to allow the layer of whiteparticles remote from the rear electrode to be readily visible to theobserver. Typically, the suspending fluid in such a display is notcolored at all, but some color may be incorporated for the purpose ofcorrecting any undesirable tint in the white particles seentherethrough.

Both single and dual particle electrophoretic displays may be capable ofintermediate gray states having optical characteristics intermediate thetwo extreme optical states already described.

Some of the aforementioned patents and published applications discloseencapsulated electrophoretic media having three or more different typesof particles within each capsule. For purposes of the presentapplication, such multi-particle media are regarded as sub-species ofdual particle media.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned 2002/0131147. Accordingly, for purposes of thepresent application, such polymer-dispersed electrophoretic media areregarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Application Publication No. WO 02/01281, andpublished US Application No. 2002/0075556, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346. Other types ofelectro-optic displays may also be capable of operating in shutter mode.

An encapsulated or microcell electrophoretic display typically does notsuffer from the clustering and settling failure mode of traditionalelectrophoretic devices and provides further advantages, such as theability to print or coat the display on a wide variety of flexible andrigid substrates. (Use of the word “printing” is intended to include allforms of printing and coating, including, but without limitation:pre-metered coatings such as patch die coating, slot or extrusioncoating, slide or cascade coating, curtain coating; roll coating such asknife over roll coating, forward and reverse roll coating; gravurecoating; dip coating; spray coating; meniscus coating; spin coating;brush coating; air knife coating; silk screen printing processes;electrostatic printing processes; thermal printing processes; ink jetprinting processes; and other similar techniques.) Thus, the resultingdisplay can be flexible. Further, because the display medium can beprinted (using a variety of methods), the display itself can be madeinexpensively.

An electro-optic display normally comprises a layer of electro-opticmaterial and at least two other layers disposed on opposed sides of theelectro-optic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electro-optic display, which is intended foruse with a stylus, print head or similar movable electrode separate fromthe display, only one of the layers adjacent the electro-optic layercomprises an electrode, the layer on the opposed side of theelectro-optic layer typically being a protective layer intended toprevent the movable electrode damaging the electro-optic layer.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts as anone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by this process isroll lamination using a lamination adhesive. Similar manufacturingtechniques can be used with other types of electro-optic displays. Forexample, a microcell electrophoretic medium or a rotating bichromalmember medium may be laminated to a backplane in substantially the samemanner as an encapsulated electrophoretic medium.

As discussed in the aforementioned 2004/0027327, many of the componentsused in solid electro-optic displays, and the methods used tomanufacture such displays, are derived from technology used in liquidcrystal displays (LCD's), which are of course also electro-opticdisplays, though using a liquid rather than a solid medium. For example,solid electro-optic displays may make use of an active matrix backplanecomprising an array of transistors or diodes and a corresponding arrayof pixel electrodes, and a “continuous” front electrode (in the sense ofan electrode which extends over multiple pixels and typically the wholedisplay) on a transparent substrate, these components being essentiallythe same as in LCD's. However, the methods used for assembling LCD'scannot be used with solid electro-optic displays. LCD's are normallyassembled by forming the backplane and front electrode on separate glasssubstrates, then adhesively securing these components together leaving asmall aperture between them, placing the resultant assembly undervacuum, and immersing the assembly in a bath of the liquid crystal, sothat the liquid crystal flows through the aperture between the backplaneand the front electrode. Finally, with the liquid crystal in place, theaperture is sealed to provide the final display.

This LCD assembly process cannot readily be transferred to solidelectro-optic displays. Because the electro-optic material is solid, itmust be present between the backplane and the front electrode beforethese two integers are secured to each other. Furthermore, in contrastto a liquid crystal material, which is simply placed between the frontelectrode and the backplane without being attached to either, a solidelectro-optic medium normally needs to be secured to both; in most casesthe solid electro-optic medium is formed on the front electrode, sincethis is generally easier than forming the medium on thecircuitry-containing backplane, and the front electrode/electro-opticmedium combination is then laminated to the backplane, typically bycovering the entire surface of the electro-optic medium with an adhesiveand laminating under heat, pressure and possibly vacuum.

As discussed in the aforementioned U.S. Pat. No. 6,312,304, themanufacture of solid electro-optic displays also presents problems inthat the optical components (the electro-optic medium) and theelectronic components (in the backplane) have differing performancecriteria. For example, it is desirable for the optical components tooptimize reflectivity, contrast ratio and response time, while it isdesirable for the electronic components to optimize conductivity,voltage-current relationship, and capacitance, or to possess memory,logic, or other higher-order electronic device capabilities. Therefore,a process for manufacturing an optical component may not be ideal formanufacturing an electronic component, and vice versa. For example, aprocess for manufacturing an electronic component can involve processingunder high temperatures. The processing temperature can be in the rangefrom about 300 C. to about 600 C. Subjecting many optical components tosuch high temperatures, however, can be harmful to the opticalcomponents by degrading the electro-optic medium chemically or bycausing mechanical damage.

This patent describes a method of manufacturing an electro-optic displaycomprising providing a modulating layer including a first substrate andan electro-optic material provided adjacent the first substrate, themodulating layer being capable of changing a visual state uponapplication of an electric field; providing a pixel layer comprising asecond substrate, a plurality of pixel electrodes provided on a frontsurface of the second substrate and a plurality of contact pads providedon a rear surface of the second substrate, each pixel electrode beingconnected to a contact pad through a via extending through the secondsubstrate; providing a circuit layer including a third substrate and atleast one circuit element; and laminating the modulating layer, thepixel layer, and the circuit layer to form the electro-optic display.

Electro-optic displays are often costly; for example, the cost of thecolor LCD found in a portable computer is typically a substantialfraction of the entire cost of the computer. As the use of electro-opticdisplays spreads to devices, such as cellular telephones and personaldigital assistants (PDA's), much less costly than portable computers,there is great pressure to reduce the costs of such displays. Theability to form layers of some solid electro-optic media by printingtechniques on flexible substrates, as discussed above, opens up thepossibility of reducing the cost of electro-optic components of displaysby using mass production techniques such as roll-to-roll coating usingcommercial equipment used for the production of coated papers, polymericfilms and similar media. However, such equipment is costly and the areasof electro-optic media presently sold may be insufficient to justifydedicated equipment, so that it may typically be necessary to transportthe coated medium from a commercial coating plant to the plant used forfinal assembly of electro-optic displays without damage to therelatively fragile layer of electro-optic medium.

Also, most prior art methods for final lamination of electrophoreticdisplays are essentially batch methods in which the electro-opticmedium, the lamination adhesive and the backplane are only broughttogether immediately prior to final assembly, and it is desirable toprovide methods better adapted for mass production.

The aforementioned 2004/0027327 describes a method of assembling a solidelectro-optic display (including a particle-based electrophoreticdisplay) which is well adapted for mass production. Essentially, thiscopending application describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this copending application and herein to mean that the layerthus designated transmits sufficient light to enable an observer,looking through that layer, to observe the change in display states ofthe electro-optic medium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present). Thesubstrate will typically be a polymeric film, and will normally have athickness in the range of about 1 to about 25 mil (25 to 634 μm),preferably about 2 to about 10 mil (51 to 254 μm). Theelectrically-conductive layer is conveniently a thin metal layer of, forexample, aluminum or ITO, or may be a conductive polymer. Poly(ethyleneterephthalate) (PET) films coated with aluminum or ITO are availablecommercially, for example as “aluminized Mylar” (“Mylar” is a RegisteredTrade Mark) from E.I. du Pont de Nemours & Company, Wilmington Del., andsuch commercial materials may be used with good results in the frontplane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

The aforementioned 2004/0027327 also describes a method for testing theelectro-optic medium in a front plane laminate prior to incorporation ofthe front plane laminate into a display. In this testing method, therelease sheet is provided with an electrically conductive layer, and avoltage sufficient to change the optical state of the electro-opticmedium is applied between this electrically conductive layer and theelectrically conductive layer on the opposed side of the electro-opticmedium. Observation of the electro-optic medium will then reveal anyfaults in the medium, thus avoiding laminating faulty electro-opticmedium into a display, with the resultant cost of scrapping the entiredisplay, not merely the faulty front plane laminate.

The aforementioned 2004/0027327 also describes a second method fortesting the electro-optic medium in a front plane laminate by placing anelectrostatic charge on the release sheet, thus forming an image on theelectro-optic medium. This image is then observed in the same way asbefore to detect any faults in the electro-optic medium.

The aforementioned 2004/0155857 describes a so-called “double releasefilm” which is essentially a simplified version of the front planelaminate of the aforementioned 2004/0027327. One form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay.

All the aforementioned methods for assembly of solid electro-opticdisplays leave at least one layer of lamination adhesive between theelectro-optic medium and one of the electrodes. This is disadvantageousbecause it is generally desirable for an electro-optic display to switchas quickly as possible, and to achieve such quick switching it isnecessary to provide as high an electric field as possible across theelectro-optic layer. The presence of a lamination adhesive layertogether with the electro-optic layer between the electrodes necessarilyreduces the electric field acting on the electro-optic layer at anygiven voltage between the electrodes, since some voltage dropnecessarily occurs in the lamination adhesive layer; in effect, thelamination adhesive layer wastes part of the available voltage. Althoughone can compensate for the voltage drop across the adhesive layer byincreasing the operating voltage of the display (i.e., the voltagedifference between the electrodes), increasing the voltage across theelectrodes in this manner is undesirable, since it increases the powerconsumption of the display, and may require the use of more complex andexpensive control circuitry to handle the increased voltage involved.

As already mentioned, in an encapsulated electrophoretic medium, theelectrophoretic layer typically comprises a binder which surrounds thecapsules and maintains them in the form of a mechanically coherentlayer. Other forms of solid electro-optic media may contain similarbinders; for example, the matrix of a rotating bichromal member displaycan be regarded as a binder, as could the end walls of a microcelldisplay. It has now been discovered that, if the properties of thisbinder, and, in at least some cases, the proportion of binder present inthe electro-optic layer, are chosen carefully, the binder can also serveas a lamination adhesive, thus removing the need for a separatelamination adhesive layer and thus producing improved electro-opticperformance in the final display.

Accordingly, in one aspect this invention provides a solid electro-opticdisplay having a binder which also serves as a lamination adhesive.

A second aspect of the present invention relates to flexible displays.Flexible display technology is highly desirable for a number of displayapplications. One application in which flexibility is critical is thecase in which the display is used above a mechanical or electricalsensing device where the response of the sensing device is produced bymechanical deformation, for example, by throwing a switch or bymechanically changing the spacing in a capacitor, or piezoelectricsensor, or other electrical or electronic device. The compliance andflexibility of the display is crucial in these applications; if thedisplay layers are too stiff, more force is required to make the sensoroperate, and the effective sensing resolution of the device is reduced,since more than one sensing element might be operated by applyingpressure at a given point. One example of an application in which thestiffness of the display assembly has been shown to be important is in atelephone keypad, where it is desired to have a display above an arrayof microswitches, operated by finger pressure. The stiffness of presentencapsulated displays coated on relatively thick plastic supports andusing plastic backplanes has been shown to complicate the assembly ofthese keypads and to reduce the tactile feel of the switching operation(a “click” on closure of the switch).

Thus, in a second aspect, the present invention relates to processes forassembly of flexible electro-optic displays; these processes make use ofcomponents somewhat analogous to the aforementioned front planelaminates and double release films.

Furthermore, as already mentioned a further aspect of this inventionrelates to color displays. One of the problems with many electro-opticdisplays is the limited range of colors which each pixel of the displaycan produce. As discussed above, both the single and dual particle typesof electrophoretic display normally only display two colors at eachpixel, the colors of the particle and the suspending fluid in a singleparticle display, and the colors to the two types of particles in andual particle display.

One approach to expanding the limited range of colors available fromconventional electro-optic displays is to place an array of coloredfilters over the pixels of the display. For example, consider the effecton a display comprising white particles in a black fluid of placing anarray of color filters (say red, green and blue) over the individualpixels of the display. Moving the white particles adjacent the viewingsurface of a pixel covered with a red filter would color that pixel red,whereas moving the white particles of the same pixel adjacent the rearsurface of the display would render the pixel black. The main problemwith this approach to generating color is that the brightness of thedisplay is limited by the pixelation of the color filter. For example,if a red color is desired, the pixels covered by red filters are set toappear red, whereas the pixels covered by green and blue filters are setto appear dark, so that only a fraction of the display surface has thedesired color while the remaining portion is dark, thus limiting thebrightness of any color obtained. A reflective display that was capableof three optical states (black, white and color or black, white andtransparent) would significant advantages in image quality, cost andease of manufacture.

One aspect of the present invention relates to the use shutter-modeelectro-optic media to produce improved color displays.

Also, as already mentioned, a further aspect of the present inventionrelates to processes and components for forming electro-optic displaysusing the front plane laminates and double release films describedabove. In a practical commercial, high volume process, it is necessaryat present to use a thermal lamination process to attach the FPL ordouble release film to the backplane. The backplane may be of the directdrive segmented variety with one or more patterned conductive traces, ormay be of the non-linear circuit variety (e.g. active matrix).

During the development of the processes to laminate the FPL or doublerelease film to glass active matrix backplanes (thin film transistorarrays, or simply TFT's), numerous problems have been encountered withtraditional lamination equipment. This invention provides modificationsof conventional tooling that are required or desirable in order tofacilitate the processing of FPL's and double release films on glassTFT's. The inventions described herein may be useful in the design oflamination tools for FPL- or double release film-based displays that useplastic or metal foil backplanes as well.

Finally, this invention relates to processes for manufacturing a hybriddisplay formed from materials having differing coefficients of thermalexpansion. Electro-optic displays may be built using two plates ofglass. The first plate forms a front surface and provides one or moreelectrodes for addressing an electro-optic medium. The second plateforms a back surface and provides one or more electrodes (and possiblynon-linear elements such as thin film transistors) for addressing theelectro-optic medium. Ideally, the materials used to form the front andback plates are similar in certain mechanical properties, such as theircoefficient of thermal expansion (CTE) and the coefficient of relativehumidity expansion (CHE). Further, in some instances, it is desirablefor the materials to have selected combinations of thickness and Young'sModulus (E) in order to satisfy certain requirements for manufacture.

In other cases, such as when a display is formed using an FPL and aglass or similar rigid backplane, the resultant “hybrid” electro-opticdisplay inevitably has its front and back “plates” of materials thatdiffer in their mechanical properties. Such hybrid displays give rise tonew challenges in their manufacture. For example, a display constructedusing an encapsulated electrophoretic FPL and a glass TFT backplane has,in effect, a plastic front plate laminated to a glass backplane in whatis fundamentally an asymmetric stack of dissimilar materials. As aresult of this construction, the display exhibits mechanical behaviorthat is not found in a traditional glass/glass display. Specifically,the asymmetric construction leads to curl (warping) of the compositepanel as a function of panel temperature or humidity changes. Thestresses and strains associated with warping place extreme challenges onthe design of such systems. Accordingly, there is a need for panelprocessing, materials, and construction methodology that lead toacceptable performance of the panel over a wide range of operatingenvironments, and the present invention seeks to meet these needs.

SUMMARY OF THE INVENTION

Accordingly, in one aspect this invention provides a process forproducing an encapsulated electrophoretic display, the processcomprising:

providing an electrophoretic medium comprising a plurality of discretedroplets in a polymeric binder, each droplet comprising a plurality ofcharged particles dispersed in a suspending fluid and capable of movingtherethrough on application of an electric field to the suspendingfluid;

providing a backplane having at least one electrode; and

laminating the electrophoretic medium to the backplane at a temperatureat which the polymeric binder will flow and with the electrophoreticmedium in direct contact with the backplane, thereby causing thepolymeric binder to flow and secure the electrophoretic medium to thebackplane to form the display.

As will readily be apparent to those skilled in the manufacture ofelectro-optic displays, this process differs from conventional processesfor lamination of electro-optic media to backplanes in that nolamination adhesive is needed between the electro-optic display and thebackplane; in effect, the polymeric binder functions as both a binderand a lamination adhesive. Accordingly, this process may for conveniencehereinafter be called the “adhesive-less” process of the invention.

In this process, the electrophoretic medium may be of any of the typespreviously described. Thus, for example, the electrophoretic medium maybe a conventional encapsulated electrophoretic medium, in which eachdroplet is confined within a capsule wall separate from the polymericbinder (although such a capsule wall may itself be formed from apolymeric material). Alternatively, the electrophoretic medium may be ofthe polymer-dispersed type, with the droplets forming the discontinuousphase of a two-phase system and being surrounded by a continuous phaseforming the polymeric binder.

In the adhesive-less process of the present invention, theelectrophoretic medium may be disposed on a light-transmissive substrateso that, following the lamination, the electrophoretic medium issandwiched between the substrate and the backplane. A light-transmissiveelectrode may be disposed between the electrophoretic medium and thesubstrate, and the electrophoretic medium may be provided with a releasesheet covering its surface remote from the substrate (i.e., theelectrophoretic medium may be incorporated into a front plane laminateas described in the aforementioned 2004/0027327) and the release sheetremoved prior to the lamination.

In the adhesive-less process, the lamination is conducted at atemperature sufficient to cause the polymeric binder to flow, so thatthe binder will flow and secure the electrophoretic medium to thebackplane. The temperature used should, of course, not be so high as tocause unacceptable damage to the electrophoretic medium or to any othertemperature-sensitive component present. Thus, the binder should bechosen so that it flows at a temperature which permits the lamination tobe effected without damage to the electrophoretic medium or othercomponent. In general, it is desirable to use a polymeric binder whichflows at a temperature of not more than about 150° C., and preferablynot more than about 100° C. As explained in detail in the aforementioned2003/0025855, the choice of lamination adhesives for use inelectrophoretic displays is complicated because a large number offactors have to be considered, including the electrical properties ofthe adhesive, and the same factors apply to a polymeric binder alsofunctioning as a lamination adhesive. Accordingly, for the same reasonsas discussed in the aforementioned 2003/0025855, it is generallypreferred that the polymeric binder used in the adhesive-less process bea polyurethane.

For reasons discussed in more detail below, the ratio of polymericbinders to droplets in the electrophoretic medium used in theadhesive-less process is normally higher than in prior art processesusing an adhesive separate from the binder. In the adhesive-lessprocess, typically the polymeric binder will comprise at least about 20,and desirably at least about 30, percent by weight of theelectrophoretic medium.

The backplane used in the adhesive-less process may be of any of thetypes known in the art. For example, the backplane may be of the directdrive type, having a plurality of pixel electrodes and conductive tracesby which the potentials on the pixel electrodes can be independentlycontrolled. Alternatively, the backplane may be an active matrixbackplane comprising a plurality of pixel electrodes and at least onenon-linear element associated with each pixel electrode.

This invention also provides an electrophoretic medium (which mayhereinafter be called the “adhesive-less medium” of the presentinvention) intended for use in the adhesive-less process describedabove. This electrophoretic medium comprises a plurality of discretedroplets of electrophoretic medium in a polymeric binder, each dropletcomprising a plurality of charged particles dispersed in a suspendingfluid and capable of moving therethrough on application of an electricfield to the suspending fluid, wherein the polymeric binder flows at atemperature of not more than about 150° C.

In this adhesive-less medium, desirably the polymeric binder flows at atemperature of not more than about 100° C. The electrophoretic mediummay be of any of the types previously described. Thus, for example, theelectrophoretic medium may be a conventional encapsulatedelectrophoretic medium, in which each droplet is confined within acapsule wall separate from the polymeric binder (although such a capsulewall may itself be formed from a polymeric material). Alternatively, theelectrophoretic medium may be of the polymer-dispersed type, with thedroplets forming the discontinuous phase of a two-phase system and beingsurrounded by a continuous phase forming the polymeric binder.

The adhesive-less medium of the present invention may be used incombination with a light-transmissive substrate covering one surface ofthe medium, optionally with a light-transmissive electrode disposedbetween the electrophoretic medium and the substrate. Theelectrophoretic medium may be provided with a release sheet covering itssurface remote from the substrate (i.e., the electrophoretic medium maybe incorporated into a front plane laminate as described in theaforementioned 2004/0027327).

For reasons discussed above, in the adhesive-less medium, typically thepolymeric binder will comprise at least about 20, and desirably at leastabout 30, percent by weight of the electrophoretic medium, and thepolymeric binder may comprise a polyurethane.

This invention also provides an article of manufacture comprising, inorder:

a light-transmissive electrically-conductive layer;

an electrophoretic medium comprising a plurality of discrete droplets ofelectrophoretic medium in a polymeric binder, each droplet comprising aplurality of charged particles dispersed in a suspending fluid andcapable of moving therethrough on application of an electric field tothe suspending fluid, the polymeric binder flowing at a temperature ofnot more than about 150° C.; and

a release sheet in contact with the polymeric binder.

This article of manufacture is in effect a front plane laminate asdescribed in the aforementioned 2004/0027327, modified to replace theelectrophoretic medium and lamination adhesive layers of the originalfront plane laminate with a electrophoretic medium having a binder whichcan also function as a lamination adhesive, in accordance with thepresent invention.

This invention also provides an article of manufacture comprising:

a layer of an electrophoretic medium comprising a plurality of discretedroplets of electrophoretic medium in a polymeric binder, each dropletcomprising a plurality of charged particles dispersed in a suspendingfluid and capable of moving therethrough on application of an electricfield to the suspending fluid, the polymeric binder flowing at atemperature of not more than about 150° C., the layer having first andsecond surfaces on opposed sides thereof;

a first release sheet covering the first surface of the layer ofelectrophoretic medium; and

a second release sheet covering the second surface of the layer ofelectrophoretic medium.

This article of manufacture is in effect a double release sheet asdescribed in the aforementioned 2004/0027327, modified to replace theelectrophoretic medium and lamination adhesive layers of the originalfront plane laminate with a electrophoretic medium having a binder whichcan also function as a lamination adhesive, in accordance with thepresent invention.

In another aspect, this invention provides a process for forming asub-assembly for use in an electro-optic display, this processcomprising:

depositing a layer of an electro-optic medium on a first release sheet;

depositing a layer of a lamination adhesive on a second release sheet;and

thereafter contacting the electro-optic medium on the first releasesheet with the lamination adhesive on the second release sheet underconditions effective to cause the lamination adhesive to adhere to theelectro-optic medium, thereby forming a sub-assembly comprising thelamination adhesive and the electro-optic medium sandwiched between thetwo release sheets.

This process, which is primarily, although not exclusively, intended foruse in the assembly of flexible displays, may hereinafter forconvenience be called the “flexible sub-assembly process” of the presentinvention. This process may further comprise removing the first releasesheet from the sub-assembly and laminating the electro-optic medium to abackplane comprising at least one electrode. The process may furthercomprise laminating a layer of laminating adhesive to the backplaneprior to laminating the electro-optic medium thereto.

In another aspect, this invention provides apparatus for displaying acolor image, this apparatus comprising an electro-optic display having aplurality of pixels, each of which can be independently set to alight-transmissive optical state or a substantially opaque opticalstate, and lighting means arranged to flash separate pulses of light ofat least two differing colors on to one surface of the electro-opticdisplay.

In a further aspect, this invention provides apparatus for generatingpulses of light of differing colors, the apparatus comprising a lightsource and a filter assembly arranged to receive light from the lightsource, the filter assembly comprising:

a first electro-optic layer having a light-transmissive state and acolored state having a first optical characteristic;

a first electrode arranged to apply to the first electro-optic layer anelectric field capable of switching the first electro-optic layerbetween its light-transmissive and colored states;

a second electro-optic layer having a light-transmissive state and acolored state having a second optical characteristic different from thefirst optical characteristic; and

a second electrode arranged to apply to the second electro-optic layeran electric field capable of switching the second electro-optic layerbetween its light-transmissive and colored states.

In another aspect, this invention provides a first method formanufacturing a hybrid display, this first method comprising:

(a) providing a front plane laminate comprising an electro-optic layerand a substrate, the front plane laminate having a first coefficient ofthermal expansion (CTE);

(b) producing an electro-optic display by laminating the front planelaminate to a backplane comprising at least one electrode, the backplanehaving a second CTE;

(c) heating the display to a temperature above a threshold temperature,thereby producing a heated display with a curvature; and

(d) gradually lowering the temperature to an ambient temperature torelease structural stress resulting from any differential expansion ofthe front plane laminate and the backplane such that the curvature issubstantially reduced.

In another aspect, this invention provides a second method formanufacturing a hybrid display, this second method comprising:

(a) adhering a front plane laminate comprising a first material having afirst coefficient of thermal expansion (CTE) to a backplane comprising asecond material having a second CTE, thereby producing a hybrid displaywith a first curvature; and

(b) reducing the curvature of the hybrid display by forcing the displayto temporarily assume a second curvature opposite the first curvature.

In another aspect, this invention provides a third method formanufacturing a hybrid display, this third method comprising:

(a) providing a front plane laminate comprising a first material havinga first coefficient of thermal expansion (CTE);

(b) adhering a backplane comprising a second material having a secondCTE to the front plane laminate; and

(c) producing a hybrid display by adhering a third panel comprising amaterial different from the second material to the backplane such thatthe overall curvature of the hybrid panel is substantially reducedcompared to a display consisting of only the front plane laminate andthe backplane but not the third panel.

Finally, this invention provides a fourth method for manufacturing ahybrid display, this fourth method comprising:

(a) adjusting a front plane laminate comprising a first material havinga first coefficient of thermal expansion (CTE) to a first temperature;

(b) adjusting a backplane comprising a second material having a secondCTE to a second temperature; and

(c) adhering the temperature-adjusted front plane laminate to thetemperature-adjusted backplane to produce a hybrid display.

In each of the aforementioned methods for manufacturing a hybriddisplay, the front plane laminate may comprise an electrophoretic layer,which may be of any of the types previously mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the accompanying drawings is a schematic section through aprior art front plane laminate as described in the aforementioned2004/0027327, but with the release layer removed ready for lamination toa backplane.

FIG. 2 is a schematic section showing a prior art display resulting fromlaminating the front plane laminate shown in FIG. 1 to a backplanecontaining pixel electrodes.

FIG. 3 is a schematic section, similar to that of FIG. 1, through afront plane laminate of the present invention, again with the releaselayer removed ready for lamination to a backplane.

FIG. 4 is a schematic section, similar to that of FIG. 2, showing adisplay resulting from laminating the front plane laminate shown in FIG.1 to a backplane containing pixel electrodes.

FIGS. 5 to 9 are schematic side elevations showing various stages in aflexible sub-assembly process of the present invention.

FIG. 10 is a schematic side elevation of an apparatus of the presentinvention for displaying color images.

FIG. 11 is a schematic side elevation of an apparatus of the presentinvention for generating pulses of light of differing colors.

FIG. 12 is a schematic section through a display cell of an hybridelectrophoretic display which can be manufactured in accordance with thepresent invention.

FIGS. 13A and 13B are schematic side elevations illustrating a methodwhich may be used to relieve warping in a display such as that shown inFIG. 12.

FIG. 14 is a schematic side elevation showing a further method which maybe used to prevent warping in a display such as that shown in FIG. 12.

FIG. 15 illustrates a method of manufacturing a hybrid display in whichtemperature adjustment is used to control any tendency for the displayto warp.

FIG. 16 is a schematic section, considerably enlarged as compared withFIG. 12, showing one possible edge seal geometry which may be used inthe display of FIG. 12.

DETAILED DESCRIPTION

As already indicated, this invention has several different aspectsrelating to electro-optic displays and to processes and components forthe production of such displays. These various aspects will mainly bedescribed separately below, but it should be understood that a singledisplay, process or component may make use of more than one aspect ofthe invention. For example, the various processes of the invention formanufacturing hybrid displays may be carried out using a front planelaminate of the invention.

Adhesive-Less Process, Medium, Front Plane Laminate and Double-ReleaseFilm

All of the above aspects of the present invention are grouped togetherbecause they all relate to eliminating the lamination adhesiveconventionally used between an encapsulated electrophoretic medium andat least one other component of an electrophoretic display; this othercomponents is typically a backplane, but in some cases the presentinvention may allow elimination of a lamination adhesive between anelectrophoretic medium and a front substrate, which provides the viewingsurface through which an observer is intended to view the display. Asalready mentioned, the elimination of the lamination adhesive iseffected by using in the electrophoretic medium a binder which can flowat the temperature used for lamination, so that in effect the binderalso serves as the lamination adhesive. The concept of flow iscomplicated but herein it refers to a polymeric material that has passedthrough the transition from elastic to plastic or viscous behavior.

The binder used in the adhesive-less medium and process of the inventionmust of course be chosen not only with regard to its flow temperaturebut also with regard to its compatibility with the other components ofthe electrophoretic medium and the requirements for driving this medium,including in particular the resistivity of the binder. Desirably, thebinder flows at a temperature of not more than about 150° C., andpreferably at a temperature of not more than about 100° C. In general,the preferred type of binder is a polyurethane; it has been found thatcertain polyurethanes can meet these preferred flow temperatures whilestill being compatible with all the other components conventionally usedin electrophoretic media.

Also, in choosing the binder to be used in the adhesive-less medium andprocess of the invention, attention should be given not only to the typeof binder used but also to the proportion of binder. As discussed inseveral of the E Ink and MIT patents and applications mentioned above,encapsulated electrophoretic media prepared by coating a mixture ofcapsules and binder on to a substrate, or media of the polymer-dispersedtype, tend to have a non-planar surface, since the individual capsulesor droplets form “bumps” on the surface of the dried and/or curedelectrophoretic medium, and this is especially true when (as ispreferably the case) the medium consists essentially of a single layerof capsules or droplets. In a conventional process using a laminationadhesive, the lamination adhesive serves not only to adhere theelectrophoretic medium to a backplane or other component, but also toplanarize the original non-planar surface of the electrophoretic medium,thus avoiding various problems (for example, the formation of voids anduneven response of the electrophoretic medium to applied electricfields) which might otherwise occur if the non-planar surface of theelectrophoretic medium is laminated to a planar surface of a backplaneor other component.

When the lamination adhesive is eliminated in accordance with thepresent invention, in order to avoid such problems, it is highlydesirable that the flowable binder replace not the adhesive function ofthe lamination adhesive previously used but also its planarizingfunction, and to enable the flowable binder to do so, it has been foundgenerally desirable to use a higher proportion of binder in theadhesive-less electrophoretic of the present invention than is typicallyemployed in prior art electrophoretic media intended for use withlamination adhesives. Consider, for example, an idealized encapsulatedelectrophoretic medium comprising a single layer of hexagonallyclose-packed spherical capsules resting on a substrate. The volumefraction of the layer occupied by the capsules is approximately 60.5percent, leaving 39.5 percent by volume of the layer to be occupied bythe binder. This suggests that a capsules to binder volume ratio ofabout 3:2 would suffice to enable the binder to fill all of the spacebetween the capsules, thus planarizing the capsule layer. Furtherconsideration of this idealized system suggests that a slightly greaterproportion of binder (for example, a capsules to binder volume ratio of1:1) is desirable to ensure that excess binder overlies the capsulelayer, thus reducing the chance that capsule walls might be damaged, andperhaps burst, during the lamination process.

However, as explained in several of the E Ink and MIT patents andapplications mentioned above (see especially U.S. Pat. Nos. 6,067,185and 6,392,785), encapsulated electrophoretic media produced in practicediffer significantly from the idealized model, in that the originallyspherical capsules become flattened into oblate ellipsoids as the layerof electrophoretic medium shrinks during drying or curing, and that, inat least some cases, as the shrinkage continues, the oblate ellipsoidscontact each other and develop planar areas of contact extendingsubstantially perpendicular to the thickness of the layer, so thateventually the capsules have substantially the form of prisms, ideallyhexagonal prisms. Similar effects are observed with polymer-dispersedelectrophoretic media. Oblate ellipsoidal and prismatic capsules occupya substantially greater proportion of the electrophoretic medium thanclose-packed spherical capsules, so that less binder is needed in theformer cases. In addition, the foregoing discussion has focused onvolume ratios, and the density of most binders tends to somewhat higherthan that of most electrophoretic media, the greater part of which arecomposed of a low density aliphatic hydrocarbon suspending fluid, sothat the proportion by weight of the binder may be somewhat higher thanthe proportion by volume. The optimum proportion of binder for use inany particular medium is best determined empirically, but by way ofgeneral guidance it may be stated that the polymeric binder shouldtypically comprise at least about 20 percent, and desirably at leastabout 30 percent by weight of the electrophoretic medium (these ratiosare of course calculated on the weight of essentially dry capsules andon the solids basis for the binder, since the binder is typically addedas a latex). Typically, the optimum ratio will be 2 to 3 parts by weightof capsules per part by weight of polymeric binder. The use of a largeexcess of binder should be avoided, since such an excess tends to“dilute” the capsules beyond the point at which they are well-packed andhence degrade the electro-optic performance of the medium.

The lamination step of the adhesive-less process of the presentinvention may be conducted using any of the techniques known in the art.Thus, for example, the lamination may be effected using a roll-to-rollprocess by passing a front plane laminate of the invention (with therelease sheet stripped therefrom) and a roll of backplanes formed on aflexible substrate, between the heated rolls of a laminator.

The front plane laminate and double release film of the presentinvention may include any of the optional features described in theaforementioned 2004/0027327 and 2004/0155857. Thus, for example, thefront plane laminate may be provided with a conductive via and a contactpad as described in 2004/0027327. The release sheet of the front planelaminate may be provided with an electrically conductive layer tofacilitate testing of the front plane laminate in the manner describedabove.

The front plane laminate of the present invention not only eliminates alayer (namely the lamination adhesive layer) from the prior art FPL butalso simplifies the overall assembly process. In the prior art processdescribed in the aforementioned 2004/0027327, an FPL is typically formedby coating a capsule/binder slurry on to a substrate comprising apolymeric film bearing an indium-tin-oxide (ITO) layer, the slurry beingcoated on to the ITO-covered surface of the film. The resultantcapsule-coated film then undergoes a first lamination in which a layerof lamination adhesive is laminated to the exposed surface of thecapsule/binder layer, and then the release sheet is applied to form theFPL. When the FPL is to be assembled into a display, the release sheetis removed and a second lamination is effected to secure the laminationadhesive to a backplane, thus forming the final display. The presentinvention enables the first lamination to be eliminated, thussimplifying the overall process for production of the display.

This aspect of the invention will now be illustrated with reference toFIGS. 1 to 4 of the accompanying drawings. As already mentioned, FIG. 1shows a prior art front plane laminate as described in theaforementioned 2004/0027327, but with the release layer removed readyfor lamination to a backplane. As shown in FIG. 1, the front planelaminate comprises a front substrate 100, formed from a polymeric filmand bearing a layer 102 of indium tin oxide (the thickness of the layer102 is greatly exaggerated compared to that of the substrate 100 forease of illustration), which will form the common front electrode of theeventual display. The front plane laminate further comprises anelectrophoretic layer comprising capsules 104 in a binder 106, and alamination adhesive layer 108.

FIG. 2 shows the structure resulting from laminating the front planelaminate of FIG. 1 to a backplane 110 containing pixel electrodes (notshown). It will be seen that, in the laminate of FIG. 2, both theelectrophoretic layer 104/106 and the adhesive layer 108 are presentbetween the front plane electrode layer 102 and the pixel electrodes.

FIG. 3 shows a front plane laminate of the present invention, again withthe release layer removed ready for lamination to a backplane. The frontplane laminate of FIG. 3 is generally similar to that of FIG. 1 but hasa thermally flowable binder 106′ and lacks an adhesive layer.

Finally, FIG. 4 shows the structure resulting from laminating the frontplane laminate of FIG. 3 to a backplane 110. It will be seen that, inthe laminate of FIG. 4, only the electrophoretic layer 104/106 ispresent between the front plane electrode layer 102 and the pixelelectrodes. Because of the elimination of the adhesive layer between theelectrodes, at any given operating voltage, the laminate of FIG. 4 willtypically switch substantially faster than that of FIG. 2.

The following Example is now given, though by way of illustration only,to illustrate a preferred embodiment of the invention.

EXAMPLE

Dual particle opposite polarity electrophoretic capsules containingpolymer-coated titania and carbon black particles in an aliphatichydrocarbon suspending fluid and with gelatin/acacia capsule walls wereprepared substantially as described in Example 30 of the aforementioned2002/0185378. These capsules were then mixed at a weight ratio of 1:1(capsules/binder solids basis) with a custom polyurethane latex binder,and the resultant slurry slot coated, substantially as described in thisExample 30, on to a 5 mil (127 μm) poly(ethylene terephthalate) (PET)film coated on one surface with ITO, the slurry being coated on to theITO-covered surface, and cured to produce a final capsule/binder layercomprising essentially a monolayer of capsules and 15-30 μm thick. Theresultant capsule-coated film was essentially an FPL of the presentinvention except that it lacked a release sheet which was not neededsince the coated film was used immediately as described below.

The capsule coated film was then laminated using a hot roll laminator toa backplane comprising a polymeric film covered with a graphite layer,the electrophoretic layer being contacted with the graphite layer, andthe resultant structure cut to a size of 2 inch square (51 mm square) toproduce experimental single-pixel displays, which exhibited satisfactoryelectro-optic properties.

Further similar experiments indicated that satisfactory electro-opticproperties could be obtained at lower binder to capsule weight ratios ofabout 1:2 to 1:3.

Flexible Sub-Assembly Process

The flexible sub-assembly process of the present invention allows forthe assembly of a very flexible and compliant display well adapted foruse in applications such as described above in which flexibility is ofparamount importance. A preferred sub-assembly process will be describedwith reference to FIGS. 5 to 9 of the accompanying drawings.

The preferred process begins with a first release sheet 500 (FIG. 5). Alayer of an electro-optic medium 502 is coated or otherwise depositedupon the first release sheet 500. Separately, a layer of laminationadhesive 504 is formed on a second release sheet 506 and laminated tothe electro-optic medium 502 so that the lamination adhesive 504 adheresto the electro-optic medium 502.

In a separate operation, as shown in FIG. 6, a backplane 508 is formedby screen printing or a similar deposition process (see theaforementioned E Ink/MIT patents and applications for appropriateprocesses for forming such a backplane) on a third release sheet 510.Separately, a layer of lamination adhesive 512 is formed on a fourthrelease sheet 514 and laminated to the backplane 508 so that thelamination adhesive 512 adheres to the backplane 508.

In the next step of the present process, the fourth release sheet 514 isremoved from the structure shown in FIG. 6, thus exposing the laminationadhesive 512, and the first sheet 500 is removed from the structureshown in FIG. 5, thus exposing the electro-optic medium 502. The tworesultant structures are them laminated together with the laminationadhesive 512 in contact with the electro-optic medium 502, thus formingthe multi-layer structure shown in FIG. 7.

In a separate operation, another layer of lamination adhesive 516 iscoated on a fifth release sheet 518. The third release sheet 510 ispeeled from the structure shown in FIG. 7 and lamination adhesive 516and the fifth release sheet 518 are laminated to the backplane surfacethereof to produce the structure shown in FIG. 8.

The next step of the process secures the structure of FIG. 8 to asubstrate, for example a pressure sensitive switching or sensing device,which has been covered with a thin layer of dielectric material toisolate the electro-optic display components of the FIG. 8 structure.This lamination is effected by peeling the fifth release sheet 518 fromthe FIG. 8 structure and laminating the lamination adhesive 516 thusexposed to the substrate 520 (FIG. 9). Alternatively, this layer oflamination adhesive, which is in contact with the sensing or switchingdevice, can be formed of a highly insulating pressure sensitive adhesivematerial, in which case it may be possible to eliminate the layer ofdielectric material. Finally, the second release sheet 506 is removed toexpose the lamination adhesive 504, which is then laminated to a topplane electrode 522, which will typically be supported on a frontsubstrate, such as a polymeric film, which acts as a protective layer toprotect the final electro-optic display. Alternatively, the top planeelectrode 522 may be formed by a coating procedure, for example bydepositing a conductive polymer on the lamination adhesive 504.

Various alternative processes may be used following the same basicpattern. For example, the electro-optic medium can be coated directly ona conductive support in the first step of the process, and hence theremoval of the second release sheet and the lamination or formation ofthe top plane electrode omitted altogether. This procedure leads to astructure similar to that of FIG. 9, but without the topmost laminationadhesive layer. Alternatively, instead of peeling the first releasesheet to expose the electro-optic medium, the second release sheet couldbe peeled instead, to yield a bilayer coating with the adhesive layerexposed. This bilayer coating can then be laminated to a conductivelayer on a thin, plastic support. At this point, the first release sheetcould be removed and the final steps of the process proceed as before.Other assembly variations can also be envisaged, so that the generalmethod can be used to construct a variety of different electro-opticdisplay structures and devices, in a variety of ways.

The construction techniques illustrated here specifically allow thepreparation of at least two novel structures that comprise anelectro-optic display device directly adhered to a mechanical sensor ortransducer. The two novel structures differ in that in one, the top ofthe display device is protected by a protective sheet (typically aplastic film), whereas the second (wherein the top-plane electrode isapplied as a conductive polymeric material, such as PEDOT) has nofurther protective layer. Which structure is preferred depends on theapplication, and the durability of the device that is required. In thecase where the mechanical sensor is activated by buttons, those buttonfaces in contact with the electro-optic device surface can be madesmooth and/or somewhat compliant, so that a protective plastic layer maynot be required. If the actuation of the sensor is to take place using afinger, or stylus or other sharp object (as in a touch screenapplication), the top protective layer will probably be required fordurability. In either case, the durability of the device will beimproved if the electro-optic medium is a polymer dispersedelectrophoretic medium.

Other methods of applying electro-optic medium can also be used. Inparticular, electro-deposition of electrophoretic capsules and binder toa patterned backplane (see copending application Ser. No. 10/807,594filed Mar. 24, 2004 (Publication No. 2004/0226820) would be aparticularly suitable method of incorporating the electro-optic medium,and would eliminate several of the lamination/delamination steps in theabove procedure. This technique would also enable the use of multiplespot colors in addition to black or white as display enhancements.

From the foregoing, it will be seen that the flexible sub-assemblyprocess of the present invention can provide an electro-optic displaydirectly coupled to a mechanical transducer, and a general method forassembling this device using a series of lamination steps. The directcoupling between the display and the transducer eliminates the necessityfor at least one relatively stiff support sheet, and improves theresolution and feel of the coupled device. One such device enabled bythis invention is a touch screen for use with an encapsulatedelectrophoretic display; another is a telephone keypad with switchablebutton indicators. This is one use of a very powerful constructiontechnique that can take advantage of the durability and flexibility ofan encapsulated electrophoretic medium.

Apparatus for Displaying Color Images and Generating Pulses of Light

As already mentioned, the present invention provides apparatus fordisplaying color images, and apparatus for generating pulses of light ofdiffering colors, the latter apparatus being intended for use as asub-assembly in apparatus for displaying color images.

As noted above, it has been demonstrated that encapsulatedelectrophoretic and similar electro-optic media are compatible withcolor filter arrays. However, there are several fundamental challengesassociated with integrating the color filter arrays, the electrophoreticor other electro-optic medium, and the drive electronics. The key issueassociated with such a display design relates to the use of sub-pixelsto form the pixels of the display. In this architecture, sub-pixels (forexample red R, green G, and blue B) must be individually addressed inorder to trick the human eye into seeing the spectrum of the basiccolors. In other words, the smallest addressable element only shows asingle color and shades thereof, i.e., a sub-pixel cannot show theentire spectrum of visible light. In a preferred embodiment, thesmallest addressable element of the display can show each of thefundamental colors (R, G, and B in our example). The apparatus of thepresent invention addresses this problem using shutter-modeelectro-optic media and a field sequential operating technique (i.e., atechnique in which the “sub-images” representing the various colorchannels of the overall image are separated in time rather than inspace, but in such a way that the eye of the observer sees the overallcolor image).

There are two approaches to using field sequential addressing toeliminate the problem described above. First, using an apparatus of theinvention for displaying a color image, one may use a color sequentialbacklight in the display (for example, the backlight manufactured byLumiLEDs Corporation, San Jose, Calif.), and employ a “shutter-mode”electro-optic optical transducer. The field sequential backlight flashescyclically through the fundamental colors of the display, say red, greenand blue, in synchronization with the shuttering speed of the opticaltransducer. By switching the optical transducer from clear to opaquewith proper timing and spatial control, full color images may bepresented to the viewer. This type of device is illustrated in a highlyschematic manner in FIG. 10 of the accompanying drawings. FIG. 10 showsan electro-optic display comprising a substrate 1000 containing aplurality of pixel electrodes (not shown), a layer 1002 of electro-opticmedium (illustrated as an encapsulated electrophoretic medium) and acontinuous front electrode 1004. The display is provided with a fieldsequential backlight 1006 which flashes red, green and blue synchronizedwith the shuttering speed of the electro-optic medium layer 1002. Thetransducer and backlight should switch quickly enough so that the humaneye temporally integrates the colors emitted by the display.

Secondly, a color sequential backlight (i.e., an apparatus of theinvention for generating pulses of light of differing colors) may beconstructed using stacked electro-optic films. These stackedelectro-optic films, which are intended for use with a “monochrome”light modulator which may be of the electro-optic type as shown in FIG.10) require only continuous electrodes, so they would have a lowmanufacturing cost. This type of device is illustrated in a highlyschematic manner in FIG. 11 of the accompanying drawings. FIG. 11 showsa stacked color sequential back reflector comprising a blue shutter modeelectro-optic medium layer 1100 (illustrated as an encapsulatedelectrophoretic medium) provided with continuous electrodes 1102 and1104, a green shutter mode electro-optic medium layer 1106 provided withcontinuous electrodes 1108 and 1110, and a red shutter modeelectro-optic medium layer 1112 provided with continuous electrodes 1114and 1116. (It will be apparent to those skilled in the art that, byproviding a voltage source capable of generating multiple drivevoltages, one of each of the pairs of adjacent electrodes 1104/1108 and1110/1114 may be eliminated.)

In a full color optical transducer such as that shown in FIG. 11, theelectro-optic media films should be stacked to optimize theirperformance. For instance, if the transparent conductor used in thefilms is particularly absorbent to a portion of the visible spectrum,then that color should appear higher in the stack than the others. Ashutter mode optical transducer, or any other optical transducer (liquidcrystal, suspended particle displays, cholesteric liquid crystal,bistable nematic liquid crystal, etc.), may be used in conjunction withthis apparatus as the monochrome optical transducer.

Although FIG. 11 illustrates the use of three stacked films, which isthe most common arrangement (either red/green/blue oryellow/cyan/magenta), this invention is not restricted to the use of thethree stacked films; in some cases, by careful selection of colorranges, it may be possible to generate useful color images using onlytwo stacked films, or more may be used to improve the color gamut of thefinal images. Similarly, in the apparatus of the invention fordisplaying a color image, the lighting means will typically be arrangedto flash separate pulses of three different colors, but a smaller orlarger number of colors may be used.

This aspect of the present invention enables full color displays to bemanufactured using shutter-mode electro-optic media, especiallyencapsulated electrophoretic media. The apparatus of the type shown inFIG. 10 allows very high color saturation and brightness, as the colorsequential LED backlights provide impressive color gamut performance andlight output. The stacked electrophoretic shutter-mode back apparatus ofthe type shown in FIG. 11 is a low power, potentially low cost designfor a apparatus for generating pulses of light.

Processes and Components for Forming Electro-Optic Displays

As already mentioned, during the development of processes to laminateFPL's to glass active matrix backplanes (typically carrying thin filmtransistor arrays, or simply TFT's), numerous problems have beenencountered with traditional lamination equipment. This inventionprovides modifications of conventional tooling that are required ordesirable in order to facilitate the lamination of FPL's on glass TFT's.These modifications may also be useful in the design of lamination toolsfor FPL-based displays that use plastic or metal foil backplanes. Themodifications can be conveniently divided into three main areas, namelytemperature control of the lamination, placement of the FPL relative tothe backplane, and design of the stage on which the lamination iseffected, and these three areas are discussed separately below.

Temperature Control of Lamination

A conventional polarizer laminator useful for liquid crystal display(LCD) manufacturing is basically suitable for lamination of FPL's tobackplanes, provided that the conventional machine is modified toinclude a system for heating of the parts to be laminated. There arepreferred ways, some not readily apparent, however, in which to heat theparts to be laminated.

It is most desirable to apply heat in such a way that the laminationbond temperature is elevated sufficiently to cause plastic flow in thelamination adhesive (which is typically a polyurethane). There are manymeasures that may be used to describe plastic flow, which will bewell-known to those skilled in this art. For present purposes, it may bestated that the bond line temperature should be equal to or higher thanthe temperature at which the lamination adhesive's bulk and shear moduliare equal (hereafter called the cross-over point). One may achieve thistemperature level simply by heating the backplane to a temperature muchhigher than the cross-over point, and applying the FPL under pressureexerted through an unheated roller or mandrel.

In an improved process, one may also heat the roller or mandrel as wellso that even more careful control of the bond line temperature may berealized. In another variant, the roller, the backplane, and the FPLitself may be heated to effect an even higher degree of control. In thislast variant, pre-heating of the FPL allows for a considerableimprovement of throughput, as the adhesive is pre-softened beforeentering the zone in which lamination takes place. In all embodiments, athermally conductive plate (copper, aluminum, etc.) may be used toenhance thermal uniformity of the assorted heating elements.

Placement Control

A traditional polarizer laminator used in the LCD industry can positionplastic films very precisely (0.2 mm to 0.3 mm placement accuracy iscommon for state-of-the-art machines) on glass substrates. When thermallamination of FPL's is considered, however, it is found that thetemperature of lamination cannot be so high that the FPL tends to slideduring the lamination process. There are four primary parameters thataffect FPL sliding:

1. lamination adhesive material choice;

2. lamination adhesive thickness;

3. lamination temperature, and

4. forces exerted on the FPL during lamination.

As an example, for the currently-preferred polyurethane laminationadhesive used at 18 μm thickness in an FPL, it is necessary for a glassbackplane to be held at a temperature of less than 85° C. to preventsubstantial sliding of the FPL on the glass during the laminationprocess (assuming normal tension on the FPL during the laminationprocess and the use of an unheated roller). For 15 μm thickness ofadhesive, sliding is acceptable the temperature is less than 95° C. (fornormal tension on the FPL during the process). Thus, the four parameterslisted above should be controlled in order to prevent substantialsliding of the FPL during the lamination process. It is desirable forsliding to be less than 1 mm, more desirably less than 0.5 mm, and mostdesirably less than 0.3 mm.

Stage Design

This invention provides two improvements for the FPL support stage on anFPL lamination tool. The first improvement is thermal control of thesupport stage. This temperature control enables pre-heating of the FPL,and hence the adhesive on the FPL, which allows for greatly increasedlamination speed. The heating may be accomplished by conductive,convective, or radiative heating. Heating of the FPL should of coursenot be so intense as to damage the FPL.

Secondly, the stage should be smoothed in such a way that it does notscratch the plastic surface of the FPL. The stage may be coated with orbuilt from polytetrafluoroethylene (for example that sold under theRegistered Trade Mark “TEFLON” by E.I. du Pont de Nemours & Co.,Wilmington, Del.) or some similar soft, non-scratching plastic material.Alternatively, the stage may be built from metal and coated or anodizedwith non-scratching surfaces. Porous stone vacuum stages are available,but these stages may increase the tendency of the tool to scratch theFPL.

This invention also provides the following further improvements in theprocess for laminating an FPL to a backplane:

-   -   providing the ability to easily adjust the starting location for        touchdown of a roller on to the FPL. (In conventional LCD-type        equipment, a roller always touches down on the edge of the film,        which is undesirable for FPL lamination. Simple adjustments to        such a machine enable change of FPL starting position and        length, but not roller starting position; providing such an        adjustment is desirable.)    -   providing a heated roller to give better temperature uniformity        and control, and eliminate the possibility of lamination        bond-line temperature drift during a run.    -   various film stage modifications, including positioning vacuum        holes to better match the final shape of the FPL; heating the        stage (which can have secondary influence on ensuring uniform        lamination temperature, although heating the roller is more        important), and providing a flat surface on the stage to        minimize scratching of the FPL.

Manufacturing Hybrid Displays

As already mentioned, the present invention provides several methods formanufacturing hybrid displays. This aspect of the invention revolvesaround a design methodology that enables the fabrication of hybriddisplays, that is, displays built using front and back surfacescomprised of dissimilar materials. This aspect of the invention isapplicable to all types of electro-optic displays.

As already indicated, manufacturing hybrid displays is extremelycomplicated because issues arise that are nearly non-existent intraditional display cell manufacture. The present invention providesmethods and devices for manufacturing hybrid displays and includes thefollowing:

-   -   1. paying constant attention to panel curvature resulting from        CTE and CHE mismatch and using stress-releasing or reducing        methods and devices to correct curvature;    -   2. carefully controlling environmental conditions during        manufacture; and/or    -   3. producing an equilibrium, zero-stress curvature that matches        the curvature demanded by the product chassis or frame.

In one embodiment of the invention, a curl-reducing method includes acreep mechanism. The hybrid display is heated to a temperature above athreshold. Then the panel temperature is lowered gradually, over anextended period of time in some cases, to release structural stressesthat result from the differential expansion between panels of thedisplay.

In another embodiment, a curl-reducing method includes reducing thecurvature of a hybrid display by forcing the display to temporarilyassume an opposite curvature and allowing the display to spring back.For example, the display may be pressed between a weight and a curvedsurface to force-compensate its innate curvature.

In a further embodiment, a third layer is adhered to the back side ofthe back panel to compensate the physical differences between the frontpanel laminate and the back panel that otherwise would lead to a greaterdegree of curvature. The extra third layer may be different from theback panel in any number of the following physical properties: CTE, CHE,Young's Modulus, and thickness.

In yet another embodiment, the front panel laminate and the back panelare adjusted to different temperatures such that when they are adheredto form the hybrid display, the curvature is reduced if not eliminated.At least one of the panels may be a web mobilized by a roller.

According to this aspect of the present invention, other aspects of themanufacturing process are also improved to reduce the risk of displaycurvature, e.g., in an edge-sealing process and framing process.

FIG. 12 illustrates an electrophoretic display cell (generallydesignated 1210), of the type shown in FIG. 20 of the aforementioned2004/0027327 and manufactured using an FPL process. The display cell1210 may be flexible, i.e., bendable or rollable without permanentdeformation. In this construction, a front plane 1212 of the display1210 is plastic and a backplane 1214 is formed of glass and providedwith a TFT array. There may be a further protective layer 1215 over thefront plane 1212. The protective layer 1215 may be a protective layeragainst ultraviolet radiation or a barrier against ingress of oxygen ormoisture into the display 1210. Alternatively, the protective layer 1215may offer extra resistance to impact, or may enhance certain opticaleffects, e.g., with an anti-reflective coating.

A layer 1216 of an electrophoretic medium lies between the front plane1212 and the back plane 1214. The electrophoretic layer 1216 may includeone or more capsules 1218 in a binder 1220 (the electrophoreticparticles are omitted from FIG. 12 for clarity). A conductive layer 1222acts as the common front electrode of the display 1210 and is disposedbetween the front plane 1212 and the electrophoretic layer 1216. In oneembodiment, the conductive layer 1222 includes a thin,light-transmissive, and conductive material, e.g., ITO, aluminum oxide,or a conductive polymer. A circuit board 1224 is schematically shown toconnect with the back plane 1214 for addressing the electrophoreticlayer 1216 in conjunction with the conductive layer 1222.

An adhesive layer 1226 may be disposed between the electrophoretic layer1216 and the back plane 1214. There may be further layers and parts inthe display 1210 for various functions, e.g., a barrier film to furtherguard against external contaminants such as moisture, a contact pad fortest-addressing the electrophoretic layer 1216, an auxiliary adhesivelayer and so on. Some of those embodiments are described in more detailin the aforementioned 2004/0027327 and 2004/0155857. A seal 1230 maysurround one or more edges of the display 1210. In one embodiment, asalready described, the layers between and including the front plane 1212and the adhesive layer 1226 are first manufactured as an FPL 1232, whichis subsequently laminated to the back plane 1214. The protective layer1215 may be considered part of the FPL if it is attached to the frontplane 1212 before the FPL is laminated to the back plane 1214. Forsimplicity in illustration, the FPL 1232 is shown in the drawings asincluding the protective layer 1215. In one embodiment, a release sheet(not shown) is temporarily attached to the adhesive layer 1226 oppositethe electrophoretic layer 1216 as manufacture of the FPL 1232 iscompleted. The release sheet is removed prior to laminating the FPL 1232to the backplane 1214.

Examples of materials useful for making the front plane 1212 and/or theprotective layer 15 include heat stabilized poly(ethylene terephthalate)(e.g., Melinex grade 504, from Dupont Teijin Films, Wilmington, Del.)and high performance borosilicate glass (1737, from CorningIncorporated, Corning, N.Y.). In other embodiments, the PET film may bereplaced by polyethylene naphthalate (PEN), polyethersulfone (PES), orother optically transparent or near-transparent films may be constructedwithout going beyond the scope of this invention.

Other display cell structures similar to that shown in FIG. 1 may beconstructed without going beyond the scope of the invention. Otherembodiments include encapsulated electrophoretic and similarelectro-optic display cells that use:

-   -   1. a metal foil back plane the surface of which contains one or        more active or non-active electronic circuits or devices,    -   2. a plastic foil back plane the surface of which contains one        or more active or non-active electronic circuits or devices,    -   3. an FPL that includes a rigid or flexible color filter array        on a surface between the electrophoretic medium and the viewer        of the display, or    -   4. a plastic or glass substrate with active or non-active        electronic circuits or devices and a color filter array (in        which case the front plane laminate effectively becomes a back        plane laminate).

Those skilled in the art of electronic display design and integrationmay readily identify other display architectures where the principles ofthe invention can be applied without going beyond the scope of theinvention.

In the display cell shown in FIG. 12, there is a marked difference inmechanical properties between the materials that make up various layersof the display 1210, especially between the FPL 1232 and the backplane1214. For instance, the heat stabilized PET material useful for makingthe FPL 1232 typically has a CTE about 18 ppm/° C. and a CHE about 7ppm/% RH (relative humidity), while the glass making up the back plane1214 has a CTE about 3.76 ppm/° C. and a CHE about 0 ppm/% RH. As aresult of the differences in these properties, the display cell 1210exhibits mechanical properties that are highly atypical. For instance,upon encounter with heat or moisture, the display panel 1210 will curldownward (into a “frown” in the side elevation of FIG. 12), and uponcooling or drying, the display panel 1210 will curl upward (into a“smile” in FIG. 12). This behavior is obviously undesirable in afinished product. Moreover, common lamination processes require elevatedtemperatures to attach the FPL 1232 to the glass/TFT backplane 1214.Therefore, without special measures, panel curl is almost guaranteedduring hybrid display manufacture.

In order to eliminate or reduce the curling/warping phenomena, severalmethods and related devices are provided to eliminate or minimize thepanel curl during and/or after manufacture. Various methods or aspectsof these methods can be combined for purpose of practicing the presentinvention.

Referring back to FIG. 12, in a first method, part or the entire displaypanel 10 is subjected to a thermal cycle in order to relieve thestresses associated with CTE and CHE mismatch at elevated temperaturesused in lamination operations during the manufacture of the panel. Inone embodiment, part or the entire display panel is heated past athreshold temperature, for example, 50° C., or 60° C. in a dry oven orheater for a first time period, e.g., 6-10 hours, after which thetemperature is ramped downward over a second and possibly longer timeperiod. The cooling/annealing process allows the polymeric binder 1216that binds the encapsulated electrophoretic materials 1218 together, andthe lamination adhesive that adheres the encapsulated electrophoreticmaterial 1218 to the substrate layer, e.g., the conductive layer 1222,to release inherent stress through a creep mechanism. It is important togive the display cell 1210 suitable time at suitable temperature inorder to adequately relax the system to a desired state. In oneembodiment, the temperature in the second time period is loweredgradually, e.g., 1-2 degrees every hour, to the ambient temperature. Therate of temperature drop does not have to be constant, and can bevaried.

FIG. 13A illustrates a second method to relieve warping, a spring-backmechanism, which may be coupled with the creep mechanisms describedabove. The curled part of the display 1210 is temporarily placed on acurved corrective surface with a curvature that is opposite to that ofthe display 1210. The curvature of the corrective surface may be smooth.In one embodiment, the corrective surface simply constitutes aprotrusion 1340 on a flat surface. The display cell 1210 is held againstthe corrective surface 1340, e.g., by a weight 1342 on the oppositeside, possibly at an elevated temperature, e.g., 60° C. As shown in FIG.13B, the display cell 1210 is forced to assume the curvature dictated bythe protrusion 1340. After a suitable time which may range from minutesto days, the weight 1342 is removed, or the display panel 1210 isotherwise released. The display panel 1210 then springs back, in somecases, gradually, to the desired shape, i.e., flat or with a prescribedcurvature. The spring-back process may also be temperature-controlled,for example, with the temperature gradually lowered from thepreviously-elevated temperature to the ambient temperature.

FIG. 14 illustrates a further method for preventing warping in a hybriddisplay such as that shown in FIG. 12. In this method, one or morelayers 1446 of one or more materials dissimilar to the backplane 1214are attached to the back side of the backplane 1214, and laminated withthe rest of the display 1210 including the FPL 1232 as one finalproduct. The material in the additional layer 1446 is selected with aparticular CTE, CHE, Young's Modulus, or thickness so that during thelamination and ensuing processes, the display 1210 exhibits no or littlewarping. With the additional layer 1446, the display 10 exhibits mostlyaxial expansion and contraction without much warping because it ismechanically a symmetric structure. The additional layer 1446compensates for the differences in CTE, CHE, Young's Modulus, orthickness between the FPL 1232 and the back plane 1214, that otherwisewould result in warping. In one embodiment, the additional layer 1446 ismade of a material used in manufacturing the FPL 1232 such as heatstabilized PET, PEN, or PES.

FIG. 15 illustrates a further method for controlling warping byadjusting, i.e., by heating or cooling, the FPL 1232 and the back plane1214 to different temperatures before, during and/or after thelamination process. In FIG. 15, a flex-on-flex architecture orroll-to-roll lamination process is shown as an example of this method. Aweb of FPL 1232, stripped of its release sheet 1233, is laminated to aweb of backplane 1214 formed on a flexible substrate. The web ofbackplane 1214 may make use of transistors formed from polymericsemiconductors, as described in some of the aforementioned E Ink and MITpatents and published applications. Such a roll-to-roll lamination maybe effected by passing the two webs through a nip between a pair ofrollers 1548 and 1550 maintained at different temperatures. By passingquickly through rollers 1548 and 1550, the FPL 1232 and backplane 1214are respectively heated or cooled to different average temperatures,causing differential expansion in the FPL 1232 and backplane 1214 beforeand/or during lamination. Following the roll-to-roll lamination process,the combined “display” web is cut to produce individual display cells1210. Differential expansion can be set in a controlled manner to effecta desired change in the overall curl of the resulting display cell 1210.

Other lamination processes can also be used. For example, in a“piece-to-piece” process, in which individual cut pieces of FPL arelaminated to individual backplanes, each piece of FPL can be pre-heatedor cooled to a temperature different from that of the backplane. Otherapplicable lamination processes include the “web-to-piece” process wherea continuous web of FPL, stripped of any release sheet, is laminated toa plurality of backplanes arranged in holders.

Because heating potentially leads to panel warping, other steps in thedisplay manufacturing process that involve heating can also be modifiedby the present invention. Because encapsulated electrophoretic media aretypically made using aqueous coating technology, the materialsinherently attract water to some extent. When water is absorbed into thesystem, however, electrical properties of the materials may adverselychange. To ensure reasonably uniform operation over a wide range ofenvironmental conditions, it is necessary to seal the display to preventmoisture ingress, for example, through edge seal and front barrier filmtechnologies disclosed the aforementioned 2004/0027327. In some of thesealing processes, heating is used and may contribute to panel warping.

In one particular edge seal geometry illustrated in FIG. 16, a sealant1652 is wicked into a thin cavity or gap around the periphery of thedisplay cell 1210. The cavity top surface 1654 is formed by the overhangof the front protective sheet 1215 over the rest of the FPL 1232, andthe cavity bottom surface 1656 is formed by the front surface of theback plane 1214. In one form of the display, the display panel 1210 issealed using a liquid adhesive that is cured by ultra-violet radiationand/or heat. The liquid sealant 1652 containing the adhesive may bewicked into the cavity using surface tension forces, but these forcesare slowed by the viscosity of the adhesive. For high manufacturingthroughput, it is desirable to maximize the rate of wicking. To do so,the following aspects of the sealing process may be controlled orimproved by:

-   -   1. increasing the panel temperature to reduce adhesive        viscosity;    -   2. increasing the temperature of the sealant dispenser head to        reduce adhesive viscosity;    -   3. depositing a bead of adhesive that is sufficiently large to        completely fill the cavity and also leave a fillet of adhesive        just outside the cavity;    -   4. maximizing cavity height (which typically ranges between 100        μm and 300 μm);    -   5. ensuring that the adhesive substantially wets all materials        that form the cavity;    -   6. making the cavity uniform in width around the periphery of        the panel, especially at corners;    -   7. using a dispensing system with XY (in the plane of the panel)        motion control and Z (height above the panel) adjustment;    -   8. using a dispensing system with XYθ motion control (θ degree        of freedom adds the ability to swivel, for example, a dispensing        needle bent with a right angle near the tip) and Z (height above        the panel) adjustment; and/or    -   9. using a dispensing system capable of tracking edge features        through machine vision and an appropriate software algorithm.

The edge seal may be formed by a capillary wicking process with thecorrect combination of cavity design, materials choices, and processcontrols. As an example, a thin cavity with nominal dimensions of 1.5 mmin width×0.22 mm in height may be filled with UV curable adhesive (e.g.Nagase Chemtex Corporation model XNR-5516, Nagase & Co., Ltd., Tokyo,Japan). Rapid wicking can be achieved when the panel is held at anelevated temperature (e.g., 40° C. to 70° C.) in order to reduce theviscosity of the adhesive. It is also preferred to control thetemperature of the panel during UV irradiation. As an example, the panelmay be held at 40° C. while cured for 300 to 400 seconds at a UV dosageof 40 mW/cm².

One of the most critical aspects of the sealing process described aboveinvolves heating the panel to reduce adhesive viscosity, which may causestresses to develop in the display materials and result in warping. Inorder to minimize or reduce such warping effect, according to one methodof the present invention, heating is limited to the periphery of thepanel 1210 where the sealant 1652 is applied. This heating may beperformed by radiative, conductive, or convective means. If bulk heatingmust be applied, then one of the other curl-relaxation methods of thepresent invention may be used, preferably before curing of the adhesive.

In some embodiments, the display panel 1210 will be fixed in a chassisor other frame in the final product to ensure that it presents a flat oruniformly curved surface to the viewer. Bending the display panel 1210after the edge seal has been formed to conform the panel to the chassisor frame is undesirable because it superposes additional stress onto thelaminates and edge seal. According to another aspect of the invention, adisplay panel is manufactured to have a “zero-stress,” equilibriumcurvature that matches or substantially matches the desired curvature ofthe finished and framed product within the range of common operatingtemperature and relative humidity. In one embodiment, the “zero-stress”equilibrium curvature is set at or near the midpoint of the operatingenvironmental range.

For example, typical operating conditions for most electro-opticdisplays range from roughly 0° C. to 40° C. and 10% to 90% RH, but insome instances operation occurs over an even wider temperature andhumidity range. Also, testing electro-optic displays usually includes athermal shock test which subjects the display to cycles of exposure tovery low and very high temperatures (e.g., −30° C. to +80° C.). Forthese wide temperature ranges, it is desirable to minimize stresses thatwill be experienced by the panel by setting the equilibrium curvature ofthe panel near the midpoint of the environmental range, e.g., around20-25° C. in temperature and 40-60% RH.

One way to produce a display panel with a stress-free curvature is toselect materials for the front and back panel with matching mechanicalproperties such as CTE and CHE. This design methodology can be used indisplay architectures other than the plastic-on-glass architecturediscussed above, e.g., metal or plastic foil back planes, color filterarray (CFA) on the front plane architectures, CFA on back planearchitectures, and so on. For example, for a stainless steel foil backplane, the CTE is about 17 ppm/° C. and the CHE is about 0 ppm/% RH. Inthis case, curling will be dramatically reduced compared to theplastic-on-glass case, but it will likely be non-zero and somecurl-reducing technique described above can be utilized. For plasticback plane displays, it is desirable to use identical materials for theFPL and the back plane. If this is impossible, and a CTE or CHE mismatchresults, the same curl-reducing techniques described above may be used.

In displays which incorporate a color filter array (CFA), the challengesdescribed herein are compounded by the need to align the CFA sub-pixelsto the addressing elements on the back plane with precision. For a glassCFA on metal or plastic foil back plane, the CTE/CHE mismatch willresult in behavior opposite to that described in the example above;heating or moisturizing results in a smile, cooling or drying results ina frown. Despite this opposite behavior, the same curl relaxation orreduction processes described above may be employed to reduce oreliminate curl in the system. However, in this architecture, it ispreferred to use the edge seal to lock the FPL in the aligned statebefore undergoing the curl relaxation processes. If the edge seal is toocompliant, the differential expansion and contraction of the front andbackplanes will result in undesirable color shifts in the display as afunction of temperature.

In a preferred embodiment, a flex CFA architecture is manufactured inwhich a flexible CFA is attached to a metal foil back plane with a CTEclosely matched to that of the flexible CFA. For instance, the flexibleCFA can be built on heat stabilized PET (CTE of about 18 ppm/° C., CHEof about 7 ppm/% RH) and the backplane can be built on stainless steelfoil (CTE of about 17 ppm/° C., CHE of about 0 ppm/% RH). The front PETfilm can be isolated from moisture by using a front barrier film. Thinfilm options include SiO₂, SO_(x), SiO, ITO, or other transparentceramic barrier film, or polymeric materials such as Aclar™ (HoneywellInternational Corporation, Morristown, N.J.). Use of this barrierminimizes expansion and contraction of the PET due to CHE effects. Withthis type of packaging, it is possible to precisely align (accuracy towithin a few microns) under controlled environmental conditions oversizable sheets (many tens of centimeters) of high resolution flex CFA(sub-pixel pitch of several hundred pixels per inch) to flexible activematrix back planes. Of course, lower resolution embodiments may bereadily constructed by those skilled in the art.

In another embodiment, the colored patterns of the CFA are builtdirectly below or directly above the TFT on a single glass substrate andthe display is viewed through the TFT and the color filter.Advantageously, this construction does not require any alignment.

In a preferred embodiment, however, the electro-optic layer is attachedto the TFT/CFA using a “double release film” as described above and inthe aforementioned 2004/0155857. According to this approach, theelectro-optic layer or film is coated to a release material and thentransferred from that release material to the TFT/CFA. The doublerelease approach attaches the front surface of the electro-optic layerto the TFT/CFA using a thin film of adhesive, and attaches the resultantstructure to the backplane using a second layer of lamination adhesive.The double release approach offers two fundamental advantages, namelythat the front surface of the electro-optic layer offers better opticsas compared with the alternative approach wherein the laminationadhesive faces the observer, and also that since there is only a thinadhesive on the side closest to the TFT, the resolution of the displayis improved due to reduced cross-talk among pixels.

Except as specifically noted above, the preferred materials forproducing electrophoretic and other electro-optic media, and thepreferred processes for forming such media, for use in the presentinvention are the same as in similar prior art media, and for anextensive discussion of such preferred materials and processes for usein producing encapsulated electrophoretic media, the reader is referredto the aforementioned E Ink and MIT patents and applications.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of the presentinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beconstrued in an illustrative and not in a limitative sense.

1. A process for forming a sub-assembly for use in an electro-opticdisplay, the process comprising: depositing a layer of an electro-opticmedium on a first release sheet; depositing a layer of a laminationadhesive on a second release sheet; and thereafter contacting theelectro-optic medium on the first release sheet with the laminationadhesive on the second release sheet under conditions effective to causethe lamination adhesive to adhere to the electro-optic medium, therebyforming a sub-assembly comprising the lamination adhesive and theelectro-optic medium sandwiched between the two release sheets.
 2. Aprocess according to claim 1 further comprising removing the firstrelease sheet from the sub-assembly and laminating the electro-opticmedium to a backplane comprising at least one electrode.
 3. A processaccording to claim 2 further comprising laminating a layer of laminationadhesive to the backplane prior to laminating the electro-optic mediumthereto, so that the lamination adhesive becomes interposed between theelectro-optic medium and the backplane.
 4. A process according to claim2 further comprising laminating a layer of lamination adhesive to asurface of the backplane spaced from the electro-optic medium.
 5. Aprocess according to claim 4 further comprising contacting the layer oflamination adhesive on the surface of the backplane with a substrate andlaminating the layer of lamination adhesive and the backplane to thesubstrate.
 6. A process according to claim 1 further comprising removingthe second release sheet from the sub-assembly and laminating thelamination adhesive, with the electro-optic medium attached thereto, toan electrode.
 7. A process according to claim 6 wherein the electrode iscarried on a substrate which acts as a protective layer to protect theelectro-optic display produced by the process.
 8. A process according toclaim 1 further comprising removing the second release sheet from thesub-assembly and coating an electrode on the exposed laminationadhesive.
 9. A sub-assembly for use in an electro-optic display, thesub-assembly comprising, in this order: a first release sheet; a layerof an electro-optic medium; a layer of lamination adhesive; and a secondrelease sheet.
 10. A sub-assembly according to claim 9 wherein theelectro-optic medium is a rotating bichromal member or electrochromicmedium.
 11. A sub-assembly according to claim 9 wherein theelectro-optic medium is an electrophoretic medium comprising a pluralityof charged particles able to move through a fluid under the influence ofan electric field.
 12. Apparatus for displaying a color image, theapparatus comprising an electro-optic display having a plurality ofpixels, each of which can be independently set to a light-transmissiveoptical state or a substantially opaque optical state, and lightingmeans arranged to flash separate pulses of light of at least twodiffering colors on to one surface of the electro-optic display. 13.Apparatus for generating pulses of light of differing colors, theapparatus comprising a light source and a filter assembly arranged toreceive light from the light source, the filter assembly comprising: afirst electro-optic layer having a light-transmissive state and acolored state having a first optical characteristic; a first electrodearranged to apply to the first electro-optic layer an electric fieldcapable of switching the first electro-optic layer between itslight-transmissive and colored states; a second electro-optic layerhaving a light-transmissive state and a colored state having a secondoptical characteristic different from the first optical characteristic;and a second electrode arranged to apply to the second electro-opticlayer an electric field capable of switching the second electro-opticlayer between its light-transmissive and colored states.
 14. A methodfor manufacturing a hybrid display, the method comprising: (a) providinga front plane laminate comprising an electro-optic layer and asubstrate, the front plane laminate having a first coefficient ofthermal expansion (CTE); (b) producing an electro-optic display bylaminating the front plane laminate to a backplane comprising at leastone electrode, the backplane having a second CTE; (c) heating thedisplay to a temperature above a threshold temperature, therebyproducing a heated display with a curvature; and (d) gradually loweringthe temperature to an ambient temperature to release structural stressresulting from any differential expansion of the front plane laminateand the backplane such that the curvature is substantially reduced. 15.A method according to claim 14 wherein the front plane laminatecomprises an electrophoretic layer.
 16. A method for manufacturing ahybrid display, the method comprising: (a) adhering a front planelaminate comprising a first material having a first coefficient ofthermal expansion (CTE) to a backplane comprising a second materialhaving a second CTE, thereby producing a hybrid display with a firstcurvature; and (b) reducing the curvature of the hybrid display byforcing the display to temporarily assume a second curvature oppositethe first curvature.
 17. A method for manufacturing a hybrid display,the method comprising: (a) providing a front plane laminate comprising afirst material having a first coefficient of thermal expansion (CTE);(b) adhering a backplane comprising a second material having a secondCTE to the front plane laminate; and (c) producing a hybrid display byadhering a third panel comprising a material different from the secondmaterial to the backplane such that the overall curvature of the hybridpanel is substantially reduced compared to a display consisting of onlythe front plane laminate and the backplane but not the third panel. 18.A method for manufacturing a hybrid display, the method comprising: (a)adjusting a front plane laminate comprising a first material having afirst coefficient of thermal expansion (CTE) to a first temperature; (b)adjusting a backplane comprising a second material having a second CTEto a second temperature; and (c) adhering the temperature-adjusted frontplane laminate to the temperature-adjusted backplane to produce a hybriddisplay.